Add name/source filter to channel/filter tree (Natural Sort mode only)

[Bubba74]

Still work-in-progress, but most of the content is up and I think ready for discussion.

• Adds new user input boxes for specifying a smart-case regex search pattern for signal names / sources in the "Channels" and "Modify & Export" views.
  • I added the filters to "Modify & Export" for consistency, but I personally don't care because I don't use that workflow.
• Limits tree size to 10k signals. Profiling showed that it would take ~1-2 seconds to clear the widget of all signals (for very large traces of course). Subsequent updates are fast, since the widget only needs to clear the previously filtered signals.
• Adds the "path" / "source" to the signal name to assist in differentiating between signals with the same name. Perhaps this should be updated to be the group name?

WIP:

• add some tests
• move 10k signal limit to user configuration files?
• add key-bind (I like Alt-F) to move focus to the search bar
• come up with a better solution for displaying a signal's path

Example Images:

[danielhrisca]

thanks