Enabled user defined key shortcuts.

The previous updates were reworked so now the game allows a player to
defined a custom set of keys to perfom each mouse action supported.

This commit contains a few modifications done by me, but the core work
has been entirely done by Season of KDE's participant Anuj Pahuja.

My modifications are basically a few refinements done to Anuj's work.
What I did:

1. Moved a #include<KAction> from gamewidget.h to gamewidget.cpp.

2. Renamed bindKeys()'s parameter to keyShortcuts. The previous
name denoted a private local pointer used in KBlocks, where all
this code derived from.

3. moveLeft(), moveRight() etc. were renamed to moveCursorLeft(),
moveCursorRight() and so on -- a more meaningful name in
KBounceGameWidget's context.

4. setCursorPosition() was reworked to include more conditions to
move the cursor, which simplified and reduced a few lines from
all moveCursor methods.

5. The methods needed for rotating the cursor and building the wall
were mixed into a single method. This way code duplication is
avoided and a more elegant sollution applied.