Improve translation management scripts

Do a few changes to make maintaning the PO files simpler:

1. Move all scripts into scripts/
2. Remove source code location information from PO(T) files. It's mostly
   useless and produces noisy diffs with lots of unimportant changes.
3. Upon updating from Crowdin, refresh POT and POs to get consistent
   formatting and up-to-date strings.
Regenerate poedit.pot and PO files
InnoSetup: don't ask for inst dir when upgrading
Remove the old HKCR\GettextFile file association

Leaving the registry key in would confuse Explorer, which would think
there are two apps registered and would ask the user which one to use.

Clean after older Poedit versions.
InnoSetup: delete old version's ICU files

After updating ICU, the filenames changed from using "52" in the version
to "54". These files are large, make sure there are no leftovers after
Poedit 1.6.
Use HTTPS for updates checking
Fix makefiles broken by i1n scripts changes

Fix breakage introduced in 9586e7fb.
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.
Update translations from Crowdin
Change version to 1.7 and update NEWS
Reformat Poedit-Info.plist as Xcode likes it.
InnoSetup: don't include Nepali UI

Doesn't have meaningfully complete translation in Poedit.
Merge remote-tracking branch 'upstream/master' into stable