Improve thin tab bar overflow image
Allow a string value to be stored with each mark (buffer)

We were already storing a string for each mark but was (redundantly) using the mark type as value.
Remove undesired space character
Simplify code to remove a bookmark
Don’t rebase mark positions when returning marks for a subset
Asking for marks of any type now return both type and value

Previously we would only return the type.
Allow a string value to be stored with each mark (document)
Associate value with a mark using: mate --set-mark «mark»:«value»
Show popover window when clicking mark with associated value

We also suppress the “add bookmark” hover image when mouse is over a mark with associated value.
Add a ‘note’ gutter image (pointing hand)
Show number for search results when holding down command key

The number appears after command has been held down for 0.2 seconds and is meant as a hint about being able to press ⌘1-n to jump to the n’th file with matches.
Clear search gutter marks if find clipboard is changed
Maximum size of gutter image are now 1.5 times font’s cap height

We still prefer the line height but fonts like Everson Mono have a (calculated) line height twice that of the cap height which makes the images appear too big relative to the line numbers (which use a slightly smaller version of the text view’s font).

Closes #1267
Remember sort column when bundles are updated

Closes #1268