Made attribute block update when switching sprite

If the sprite you switch to doesn’t have an attribute or variable, it
will set the attribute to the default one for sprites or the stage.
Shortened the code a little
Changed "allowed" to "valid"

I think it makes a little more sense.
Use translated default values
Revert "Use translated default values"

This reverts commit 0c7de21dc3b1092d055b9bf294280a64431f856b.
Fix hiding the 'say' bubble WRT number formatting

When checking whether to hide the speech bubble, test if this block is the block that originally displayed the bubble rather than testing to see if the text matches. This avoids problems due to differences in text formatting or changes in a reporter's value.
Use GestureHandler as source for bubbles it creates

Shane explained that it would make more sense to use the GestureHandler
as the source for these particular TalkBubble instances.
Added Letters category to media library
Merge pull request #254 from nathan/letters-category

Added "Letters" category to media library
Fixed setting slider min/max not triggering autosave
Fixed pressing keys while drawing with the pencil
Fixed resizing zoomed comments
Fixed changing variable name in set/change blocks in palette
Added colons to the titles of local list watchers
Fixed alignment of drop-down menus when blocks are zoomed
Preserved position of top left of scripting area when zooming
Fixed grid for selection and stamp tools in bitmap editor
Fixed setting x or y position to +-Infinity
Fixed block not resizing when switching variable via context menu
Fixed getter blocks not resizing when renaming variable