Fix multiple warnings

 - Replace 'typedef reg' by 'struct stlink_reg'
 - Prefix the 'sl' global variable with a 'g'
 - Rename 'j' -> 'k' and 'i' -> 'j' variables
 - Rename 'hex' variable into 'hextmp'
 - Fix unused parameter
 - Use a correct printf format for gsize
 - Use correct MAX macro
 - Use correct data casting
 - Do not use 'GTK_STOCK_*' anymore
 - Do not use 'gtk_tree_view_set_rules_hint'
 - Remove unused functions
 - Explicitly cast data type
 - Enable Werror