Remove unused BasicButton::__textColor

Gets rid of an extra SystemPalette instance that installs
an event filter on the application object.

Change-Id: Ib7e1fd5634084a2b949bb5b7146fabe89f05be38
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Kill unnecessary SystemPalette instances

Each SystemPalette instance installs an event filter on the
application object, so we should try to avoid these as much as
possible. Style already creates an instance that switches between
Active and Disabled states based on control's enabled state and is
suitable for most use cases.

Change-Id: Icdbc793f639a3217ece6b2ded281e2a8beacb742
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>