[length_notifier] Add character counter

This adds a configurable character counter next to the message input.

The config dialog was based on a deprecated Gtk.Table, so I took this as an opportunity to rework the dialog.

added 2 commits

• 66ae67f6 - [length_notifier] Add character counter
• 98e90060 - [length_notifier] Update manifest.ini

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added 7 commits

• 98e90060...4fadcb53 - 5 commits from branch gajim:master
• 048e96fd - [length_notifier] Add character counter
• ba0b1129 - [length_notifier] Update manifest.ini

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merged