[proof-of-concept] Add translation_context to Control nodes

[jkirsteins]

Add a translation_context property to Control nodes.

Proof-of-concept for godotengine/godot-proposals#8061

I put together a proof-of-concept of how this could be implemented: #4

A concrete example

Before the change I use the string General in two places. A label in a settings screen (as in General settings) and on a card (as in "this is a card of an army General").

Before the change the generated *.pot file (the comment shows that it's used in two files):

#: card_template.tscn scenes/main_menu/settings_window.tscn
#: cards/card_data.tres
msgid "General"
msgstr "??? conflicting translation ???"

After the change you can specify the translation context on the label right in the Editor:

And the generated *.pot/*.po files now have two items for General, which can be translated separately:

#: card_template.tscn cards/card_data.tres
msgid "General"
msgstr "Le général"

#: scenes/main_menu/settings_window.tscn
msgctxt "settings_window"
msgid "General"
msgstr "Paramètres généraux"