New Taskview

[diegogangl]

This is a PR for the new taskview widget.

This widget is a complete rewrite from scratch of the old taskview, based on @nekohayo's idea of using a timeout to process changes.

Changes

• Waits 250ms after the last keypress before processing the text buffer
• Goes line-by-line, using regexes and simple Python functions to detect tags, etc. (way faster)
• Also needs to register less handlers
• Our custom Gtk Text Tags are now subclassed and stored in a separate module (text_tags.py)
• Tags with a custom color now have that color as a background, so they are more recognizable
• Tags can be clicked now. Clicking the selects that tag in the task browser
• Subtasks now have checkboxes that can toggle the done/not-done status (Man this was hard to get right)
• When running with -d, the process function now prints the time it took to process the buffer. This will helps us stay conscious of the performance.
• Supports internal links in the format: gtg://faf02f75-1de2-4370-8afc-34cf99e4b8fb. Of course nobody is going to use that, but we can build a proper UI down the road
• Supports invisible characters that turn visible when the cursor passes over them. This could be useful for more tags in the future
• Subtasks are added automatically and don't wait for enter to be pressed
• Autoconverts contents to the new format
• Closes the parent task when clicking on a subtask
• Supports subheadings (with # Subheading)
• Includes the foundations for more markdown features
• Completes dark mode

closes #270
closes #150
fixes #45
ref #449