Trim left/right read end in the view

Add the following two items into the "Edit" menu of the Sanger Reads Editor:

• Trim left end
• Trim right end

The item should be enabled only when a character is selected in the Reads Area and there is a character at the left/right part of the sequence.
The items should trim a read correspondingly (characters and peaks) not including the selected character.

Add the following hotkeys for the actions:

• Ctrl+Shift+Backspace
• Ctrl+Shift+Delete

Don't forget about undo/redo!