Move customized text controls into their own file

Move the various wxTextCtrl classes from the enormously huge edframe.cpp
into text_control.{h,cpp}.
Fix highlighting of text under wxGTK
Remove TextctrlHandler

Move the logic for plain-text copy & paste on Windows into
CustomizedTextCtrl.
Windows: fix undo broken by syntax higlighting.

Syntax hilighting pretty much completely broke Undo/Redo, because the
highlighting changes were considered part of the changes to record. To
fix this, suspend Undo recording while applying style changes.

This is still suboptimal, because it causes undo to work on individual
characters, rather than continuous blocks of text, with no easy
workaround. But it's better than the previous state.
Move OS X undo handling to TranslationTextCtrl

It doesn't really belong into the base CustomizedTextCtrl, so move it
into the editable text control.
Make undo work with suggestions etc.

Reorganize the code to distinguish between large edits that change the
entire text control via some edit operation such as selecting a
suggestion, copying or clearing the transaction, and between SetValue()
calls that change the displayed value programmatically when changing the
displayed item.

In the former case, the operation should be recorded in the undo stack.
In the latter, it should reset the undo stack and start anew. And that's
exactly what the code does now.
Use a bit larger default size on larger screens
Switch back to using own fork of Sparkle

So, using upstream Sparkle as a submodule was good while it lasted...
which wasn't long. Recent versions of Sparkle break Xcode archiving
workflow and the relevant bugs and PRs are still open, so a
resolution doesn't seem likely in foreseeable future.

Use own fork mostly identical, with tonyarnold's SKIP_INSTALL pull
request merged in.
See https://github.com/sparkle-project/Sparkle/issues/419 and
https://github.com/sparkle-project/Sparkle/pull/446

Mostly reverts 231967a8ab75dc2608fde6aae1a12e1d6341f501.