Log domain support and minor fixes

[xanathar]

Hi, I know it's a LOT of changes, but I think these will improve the library a lot (plus, they are backward compatible with the current implementation).

• Adds support for the log domain and the possibility to create a custom logger
• Domain can be either globally specified, inferred from context, or inferred from a G_LOG_DOMAIN static of the current module (using g_trace! ... gerror! macros, or redefined trace! ... error! macros hidden behind a feature, so not to clash with log ones unless intended).
• Adds the possibility to create a simple logger which does not print file/line info
• Serializes test execution to avoid race conditions which lead to random failures (this is caused by tests not being thread-safe because of glib_sys::g_log_set_default_handler calls which mutate global state)
• Adds unit tests using the above macros and the classical log macros
• Splits the content in several files for readability.
• Adds examples for all the features
• Adds cargo doc for all pub items

[xanathar]

This is ready for review.