Renaming Items to Usables in preparation for Skill feature
Generalizing Item to Usable type

As the upcoming Skill type will rely on many of the same foundations as
Items (object prototypes with context-sensitive "use" scripts), a lot
of the Item and ItemScript code is reusable. Renamed Item to Usable to
account for this change. The expectation is that Skill will be a
subclass of Usable. As Item is functionally equivalent to Usable right
now, it has been turned into a typedef, thus leaving a lot of the code
that depends on Item relatively intact. Item.h was created as a
convenience header when Item itself is needed specifically.
Renaming ItemData files to Metadata files.
Adding Skill Metadata and Skill Scripts

Updated the Metadata class to parse and store skill data. Also added
Skill scripts to the ScriptFactory and a test script.

Finally, updated item IDs to use a typedef called ItemId instead of
just an "int". Made the new type an unsigned int (since at this point 0
is considered invalid data, and the ID should never be negative).
Introduced SkillId for skills.
Adding Skills to Characters, adding skill menu

The skill menu was created to show a list of a character's skills. To facilitate this, the change also includes work in the HomeViewModel to handle character-dependent menus. When the Skills option is clicked in the sidebar, a character needs to be selected by clicking on one of the characters in the home pane. This work can later be extended for Equip and Status menus.

Fixed some issues with menu RCSS where the menu sidebar took up the width of the screen (though the visible area was correctly just the left 20% of the menu). This would cause the menu to appear unresponsive after a sidebar option was clicked, as the sidebar document was now in front of the menu pane. This change fixes the issue by moving some of the styles for the sidebar panel to the body of the menu shell document.

Also added some test skill data to the savegames. Assigned the two test skills, Heal and Fireball, to the character "kain". Added skill parsing and serialization into the Character class.
Fixing XCode project generation

Tweaked the CMake script to avoid producing subfolders for XCode, since
it seems to produce output that XCode cannot parse.

While verifying the build via XCode, the IDE uncovered a couple of
potential issues that I've also fixed along with this change.