v0.15: Support of custom, java implemented components

- Added the possibility to use custom, java implemented components in Digital.
- Added an EEPROM which behaves like a memory that can be written and whose content
  is non-volatile.
- Added the possibility to map keyboard keys to model buttons.
- Some small usability improvements:
  - Added a grid to the main panel.
  - Replaced shortcut 'B' with a more general attribute editing dialog (select multiple
    components and click right).
  - Added some new shortcuts (CTRL-N, CTRL-O, CTRL-A, CTRL-D).
  - Added a spinner to the input value edit dialog.
- Bug fixes
  - fixed bugs in some 74xx circuits (74160, 74161, 74162 and 74238)
  - fixed a bug in the remote interface "run to break" method.
  - fixed an error in VHDL export if comparator is used in "signed mode"
  - fixed a Windows specific bug in the speed test GUI
  - fixed a bug which causes a freezing when a file is stored in folder which contains
    a large number of sub folders and files.
- Breaking changes:
  - Removed the address bits settings from the graphic RAM. The width is now
    determined by the width and the height of the screen.