Make modifiers configurable

As pointed out in !81 (merged) the highlighter modifier (ctrl) conflicts with the synctex edit modifier. This MR aims to make both modifiers configurable and to avoid the conflict when using the default settings by changing the default highlighter modifier to shift (as to not upset the current users of ctrl+click for synctex edit).

Proposal

• new synctex-edit-modifier and highlighter-modifier options
• scope of configuration: shift, ctrl, alt
• default options: synctex-edit-modifier set to ctrl, highlighter-modifier set to shift

Also, I have simplified the logic in cb_page_widget_scaled_button_release and made some minor error formatting changes to guard against typos.