Add preference values and gui tab for changing the memory setting at launch

Jalview now has two parameters that can be used to set the max heap size of the JVM that Jalview runs in:

jvmmempc (maximum percent of physical memory to use)
jvmmemmax (maximum absolute amount of memory to use)

If one or other is not specified then they default to
jvmmempc=90
jvmmemmax=32G
although this does not necessarily dictate the memory setting (allowances are made to leave memory for the OS and a minimum requirement for the application), and if physical memory can't be ascertained when requested, the jvmmemmax will default to 8GB to be safe.

Whichever is lower (at runtime) is used.

Presently these args can be specified on the command line, or as System properties or in the .jvl file.

- jalview.bin.Launcher and com.threerings.getdown.data.Application should be changed to also look in the Jalview preferences file for other settings (if none of the above methods is used).

- The Jalview preferences pane should have a Memory Settings tab added to allow the user to set these values from within Jalview.

- Consider adding a "Restart Jalview with these settings" button???