Tracker: Add zoom shortcuts to menuless windows

(for instance the desktop)
Keymap: Add a method to get modified characters for a key

This method fills out the passed-in BList of modified utf-8 characters for
a given utf-8 character and set of modifiers.

For example if you pass in "=" and B_SHIFT_KEY the list will get filled
out with each character in the shift map that has "=" in the normal map.

Each supported keymap modifier combination is available.

The reason this is useful will soon become apparent.

A BList is used because the character might be mapped multiple times,
for example if you have a Mac keyboard you've got two "=" keys, one in
0x1d and one in 0x6a.

The caller is responsible for creating the BList and destroying it as well as
freeing the resulting character strings.
Tracker: Add semantic zoom in shortcut.

Fixes #6692

iif '+' is localed in shift + '=' in your keymap, add a shortcut for
Command + '=' to zoom in.
WebPositive: Add semantic zoom in shortcut.

Fixes #7428

iif '+' is localed in shift + '=' in your keymap, add a shortcut for
Command + '=' to zoom in.