Open/Copy menu enhancements

Ability to open/copy filepath, image or browse folder for uploads that
have errors.
fixed #98: Added drag & drop box
Drag & drop window singleton and hotkey support
Drag-and-drop window changes color

Drag-and-drop window changes color when the user drags a file to the
region.
fixed #99: Using post request for Yourls url shortener
Added drop window settings
runas needed for installer
Conditional activation of runas for installer

runas will only activate when ShareX is installed in the default Program
Files folder. If a Standard User user has installed ShareX in My
Documents, it is not necessary to execute runas.
About button was not visible on main form
fixed #102: Added ruler tool
Added mark center of ruler
Added MathHelpers, angle info to ruler
Added px unit to width, heigth and distance
Update README.md
Secondary uploaders if primary uploaders fail

Support for secondary uploaders if primary uploaders fail
Retry changes
retry changes

new enums will be added, non-existant enums will be removed
attempting visual aid

attempting visual aid for secondary uploader drag reprder
Fixed secondary uploaders reorder

DoDragDrop was disabling MouseMove Events
simplified MyListView to take care of reordering