[WIP] Add interactors functionality

[kjue25]

Changes made:

• Implements GTextField constructor, ability to get text, and adding a callback event for return key.
• Fixes issue with clearing the window erroring when interactors are on the screen.
• Adds a GWindow method for just clearing the canvas (calls existing backend function).
• Remove need for GEvents for interactor callbacks since these functions can be associated with a specific interactor in TK (in Java, event needed to distinguish which interactor the event occured on)

Decisions about general conventions/structure:

• Tell students that interactors in NORTH/SOUTH will get added left to right while WEST/EAST will get added top to bottom. They must add them in order when calling add_to_region().
• Have campy functions parallel tk as much as possible
• GInteractors shouldn't need events passed into their callback functions (unnecessary for student functionality with TK)

Currently working for NameSurfer (program not included in PR).

---STILL IN PROGRESS---
Need to fix:

• GTextField: string passed in doesn't currently do anything
• Add scrollable text/label object specifically for interactor regions (e.g. for searching name substrings in BabyNames)

Nice to have:

• Ability to center interactors within a region (maybe specify property align=left/center/right for the region?)

[sredmond]

Thank you for submitting a PR! Can you comment on the purpose of the new attribute label?

Regarding the other questions:

• GTextFields show up in Region.NORTH from the lack of lazy (Tk) instantiation throughout all of backend_tk.py (i.e. Tk constructors are called at Python object initialization time, not when the object is added for the first time).
• There is no GWindow.clear_canvas written yet, although I'd welcome one! For window.clear(), what is the error? I have a feeling it's the ordering between clearing the window and clearing the contents of the top GCompound, but I wouldn't be sure.

[sredmond]

If students shouldn't know about / stumble upon modifying this variable, it can be marked as self._label (indicating that it's intended as a 'private' variable).

[sredmond]

Small typo?

Suggested change

	gtextfield._tkin = win
	gtextfield._tkwin = win

[sredmond]

This call exposes the Tk event to the student function passed to onenter, and should probably be wrapped up into a GActionEvent eventually, although I'm not sure that the event pipeline is ready for that yet, so this is an acceptable workaround!

[sredmond]

Add this function also to backend_base.py (so that other backends know that they would have to implement it)

[sredmond]

What is the difference between text and label on this object? It looks like label is being used as a tk-based string tag for the object in gtextfield_constructor (for eventual events, perhaps?). If they are the same, these attributes should be merged into one.

[kjue25]

Here's the window.clear() error:

Traceback (most recent call last):
File "namesurfer.py", line 131, in <module>
    graph = NameSurfer(db)
File "namesurfer.py", line 51, in __init__
    self.draw_fixed_lines()
File "namesurfer.py", line 81, in draw_fixed_lines
    self.window.clear() # FIXME: Breaks with buttons already onscreen
File "/Users/kyliejue/Documents/Stanford_Docs/CS399/campy/campy/graphics/gwindow.py", line 381, in clear
    _platform.Platform().gwindow_clear(self)  # Remove from background layer.
File "/Users/kyliejue/Documents/Stanford_Docs/CS399/campy/campy/private/backends/tk/backend_tk.py", line 223, in gwindow_clear
    gwindow._tkwin.clear()
File "/Users/kyliejue/Documents/Stanford_Docs/CS399/campy/campy/private/backends/tk/backend_tk.py", line 138, in clear
    child.destroy()
AttributeError: 'str' object has no attribute 'destroy'

[kjue25]

Resolved window.clear() error above but not sure if maybe there's a cleaner solution