Make WaveEdit easier to package for Linux

[SpotlightKid]

Hi, I just created an AUR package for the Git version of WaveEdit:

https://aur.archlinux.org/packages/waveedit-git/

I would like to create a package for the release version as well, but before that is possible, a few improvements would be necessary to make live for Linux distribution packagers easier:

1. Merge Fix include paths and libs for Linux compilation #18 to fix compilation on Linux and allow to link to system libraries.

2. Adapt paths to image, font and manual PDF files used by WaveEdit to the install location at build / install time. These filenames/paths are currently hard-coded with paths relative to the executable in src/ui.cpp. Installing data files next to the executable is a no-no on Linux and the current path with which WaveEdit runs will not be where it is installed anyway.

   Ideally these paths would use a common "data path" prefix (e.g. /usr/share/waveedit), which would be set by the configure script, as is common on POSIX-compatible systems.

3. A release should have a source distribution archive (attached to the release on Guithub and/or uploaded to WaveEdit's website), with all git sub-modules included and the version number of the release somewhere in the source (currently it is in the Makefile, which is ok) and in the file name (e.g. WaveEdit-1.1.tar.gz). The archives, which Github creates automatically for each tagged release are not suitable, since they do not contain the sub-modules.

I hope you can implement these changes and if you release a new version with these changes I will make an AUR package for it asap.

[SpotlightKid]

FWIW, I'm maintaining my own fork of WaveEdit now at https://github.com/SpotlightKid/WaveEdit,
which I use as the basis of the AUR package.