Small changes to the patcher ui and typo fix

Modified the error message for missing Oxide.Core.dll to specify this
file instead of Oxide.dll
Changed the patching button to be disabled when a project has yet to be
loaded.
Added OnGather hook.

New hook that's triggered when a player gathers any of the resources.
Syntax: OnGather( ResourceDispenser source, BaseEntity player,
ItemAmount item )
Fix for opening multiple of the same tab

Fix for https://github.com/RustOxide/Oxide-2-Patcher/issues/12
Merge pull request #24 from Mughisi/master

Fixes for issues #7, #9 and #12, and OnGather hook.