Some webpages (Youtube comments for example) use <div> as text input so isContentEditable needs be checked in addition.

Relevant minor changes:
- Allowed double click to trigger popup in text input elements since it won't interfere with text editing like hotkeys do.
- Added a mouse point save before getTranslation() to avoid the triggered popup displaying in somewhere else when the network is slow and the mouse is moved.

- Rectified `exitKey` not being verified (my bad).
- Added condition for the timeout to prevent triggering `onSelectionEnd` if there's no text selected.