Reinstate timer-interrupt audio.

This reverts commit 0d048d05ee50625f15d5235d362df6ee71903fc5.
Reinstate the interrupt time setting.

This reverts commit 27d2911fb09ad81849c5df1958143c6878589450.
Lower the timer interrupt interval to 13.3 milliseconds to avoid underflows (silences).
cppcheck: [spc700.cpp:148]: (error) Uninitialized variable: buffer
cppcheck: [nds/gui.c:3573]: (error) Possible null pointer dereference: current_option - otherwise it is redundant to check if current_option is null at line 3576
cppcheck: [nds/gui.c:3130]: (error) Array 'tools_options[2]' index 4 out of bounds
cppcheck: [nds/gui.c:3132]: (error) Array 'tools_options[2]' index 4 out of bounds
Remove some unused functions. Raise the CPU frequency for sound mixing if auto frameskip is delaying because it's early.
Fix multiple compiler warnings: forward declaration, implicit declaration, unused variable, variable used uninitialised, unused function (when not used anywhere else with a #define).
Revert "Reinstate timer-interrupt audio."

This reverts commit 944d9a729455de435c3d200752308ceedf250196.

Conflicts:

	source/nds/entry.cpp
Revert "Reinstate the interrupt time setting."

This reverts commit 1bc1dfc1abf71795c43fe7abb2db487a70c524b2.

Conflicts:

	source/nds/ds2sound.h
Return to CPU level 0 after emitting sound if we're delaying for early frames.
Fix a bug whereby "state #0" is selected as the default saved state in the saved states menu if there were no saved states before. Electing to load #0 messed with the game; electing to save #0 erased the game-specific configuration.
Make many of the buttons be glass images. Darken the lines in the 6-line background for visibility.
Fix alignment and transparency problems in the previous commit's images.