Add GTK and Qt backends

[kattni]

Adds GTK 3 and Qt backends for CodeEditor, bringing the widget to Linux.

• GTK: Gtk.TextTags for highlights; the gutter is drawn into the text view's left border window. GTK 3 only, since Toga 0.5.7 has no working GTK 4 multiline input.
• Qt: a QPlainTextEdit with a QSyntaxHighlighter for the spans and a child widget for the gutter.
• Entry points for toga_gtk and toga_qt; both modules join the coverage omit list. No new dependencies.
• The example app gains a Linux Briefcase table; the README documents the Linux packages and the Qt recipe (TOGA_BACKEND=toga_qt, since both backends register as linux).

Both backends were checked on Ubuntu 26.04 with real keyboard and mouse input: highlighting of each sample, typing, a trailing newline, emoji offsets, wrapped lines, gutter width and toggling, and scrolling.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: Claude Fable 5.1 (Claude Code)

[freakboy3742]

I've made some tweaks to how the example editor is launched, so that Qt is easier to test - there's probably some more we can do to explicitly list the system dependencies, but for now, this is good enough. Thanks!