Change image annotation button tooltips

[robertknight]

Change the tooltip "Rectangle annotation" to "Rectangle selection", and change "Pin annotation" to "Pin selection".

[robertknight]

The current tooltips for controls in this toolbar are:

1. Annotation sidebar (toggle button that opens/closes the sidebar)
2. Show highlights
3. New page note (when text is not selected)
4. New annotation (when text is selected)
5. Rectangle annotation
6. Pin annotation

The tooltip for the "show highlights" toggle control follows the guideline to label controls based on what the control does when turned on.

Some notes:

• The tooltip for (1) should include a verb, "Show annotation sidebar"
• The "new annotation" button is perhaps ambiguous because it doesn't say what will be annotated. "Annotate selected text" might be better? More generally the meaning of this control is "annotate the current selection".
• It would be useful to have a concrete guideline around when to use noun versus verb phrases. The convention in the current names is to use nouns except when that would be unclear.