sys/build: Properly support per-system metadata file paths (#181)

* sys: Move away from metadata paths next to executable in the application

Build system doesn't properly install / pack stuff yet

* build: Updated README to contain better install instructions

* sys: Search for imhex resource files in ~/Application Support

* sys: MAX_PATH -> PATH_MAX

* sys: Seach for imhex resource files in Application Support using NSFileManager (#180)

* sys: Allow for multiple file search paths

Also use install prefix instead of just /usr on Linux

* build: Fixed IMHEX_INSTALL_PREFIX macro definition

* build: Fix duplicate switch entry on Linux

* docs: Updated readme to properly reflect new paths and dependencies

* sys: Install files in their proper paths on linux (#183)

* Install files in their proper paths on linux

* Only create user directories

* Follow the XDG specification on linux

XDG specification specifies how to find config and data directories on
linux systems. Specifically, it says this:

- Data should be written to $XDG_DATA_HOME
- Config should be written to $XDG_CONFIG_HOME
- Data should be read from $XDG_DATA_HOME:$XDG_DATA_DIRS
- Config should be read from $XDG_CONFIG_HOME:$XDG_CONFIG_DIRS

The default values are this:

- XDG_DATA_HOME: $HOME/.local/share
- XDG_CONFIG_HOME: $HOME/.config
- XDG_DATA_DIRS: /usr/share:/usr/local/share
- XDG_CONFIG_DIRS: /etc/xdg

Platforms with non-standard filesystems (like NixOS) will correctly set
up those environment variables, allowing softwares to work unmodified.

In order to make integration as simple as possible, we use a simple
header-only dependency called XDGPP which does all the hard work for us
to find the default directories.

* Look for plugins in all Plugin Paths

If the plugin folder was missing from one of the PluginPaths, we would
immediately stop loading plugins. We now keep looking even if one of the
path is missing.

Co-authored-by: Nichole Mattera <me@nicholemattera.com>
Co-authored-by: Robin Lambertz <unfiltered@roblab.la>

.gitmodules

@@ -5,3 +5,6 @@
 	path = external/yara/yara
 	url = https://github.com/VirusTotal/yara
 	ignore = dirty
+[submodule "external/xdgpp"]
+	path = external/xdgpp
+	url = https://git.sr.ht/~danyspin97/xdgpp