Add symbol filter to outline pane

[jeremypw]

The is a speculative step towards fixing issues such as #933, #809 and generally increasing the utility of the symbol pane over just searching the text.

• Add a toolbar header
• Search Entry in toolbar (currently only search within)
• • Option to search start only (?)
• Add Filter menu
• • Add SymbolType enum and classify Vala and C symbols with symbol type
• • Allow filtering by symbol type
• • Add Action bar to popover for bulk select/deselect filter options
• • Better icon for filter menu?
• Choosing e.g. "method" should automatically choose "class" (or "class" always included in filter?)

Note: To show class methods both the "Class" and "Methods" selections must be active.

No filtering:

Filtering class properties

Searching for text "filter"

[jeremypw]

Regression found in global search - investigating.

[jeremypw]

Looks like the regression is due to recent changes to master 😞 not this PR.

[jeremypw]

Regression now fixed by #1254. Leaving this as draft as that PR should be prioritised.

[zeebok]

It appears to work as intended!

[jeremypw]

@danirabbit Is this something worth progressing? I wasn't sure what icon should be used for the "Select all" button (or whether such a button is needed)

[danirabbit]

@jeremypw sorry for taking a while to follow up on this! I pushed a commit with the filter icon we're using in Mail and fixing the sidebar background styles.

I think for the popover I would just a regular menu here and use regular text menu items for the "select all" and "deselect" all items since those are probably more clearly communicated with text and we're not particularly constrained for space in a menu

[danirabbit]

Works as expected afaict. Nice job :)