Improve reliability of tab creation

We now make sure that Terminal is active before using the tab creation shortcut. However there is still an issue if the user hasn't released the modifier keys used to initiate the command.
Revert default and setting to TM_TERMINAL_USE_TABS

Due to lackluster AppleScript support in Terminal it is not possible to reliably open a new tab as you get an error if the user has not yet released the modifier keys required to initiate the command. This reverts c1077db238408950ed4c53b258215137eb6f3492 from pull request #20.