"Execute commands" -> "Comandos de ejecución"

"Execute commands" (from the Set Build Commands dialog) is translated to Spanish as if it were a sentence (like "Execute these commands"), but it's actually a name ("<language> commands", "Independent commands", and "Execute commands"; so it means "Commands for 'execute'").  Therefore, +"Comandos de ejecución" is a better translation than -"Ejecutar comandos".
Merge pull request #372 from cousteaulecommandant/patch-1

"Execute commands" -> "Comandos de ejecución"
Introduce GEANY_EXPORT as G_MODULE_EXPORT wrapper

The wrapper is because G_MODULE_EXPORT can't be used if/when we want to
switch to hidden-by-default visibility and only export explicit functions
because G_MODULE_EXPORT is always empty on Linux.

Change glade callbacks to use GEANY_EXPORT. Also remove it from the declaraitons
so that it's only at the definition.
api export: Dynamic export of document functions
api export: Dynamic export of scintilla functions
api export: Dynamic export of sci_*  functions
api export: Dynamic export of templates_get_template_fileheader function
api export: Dynamic export of utils functions
api export: Dynamic export of uiutils functions
api export: Remove support_lookup_widget() from GeanyFunctions.

The existing backward-compat pointer to redirect to ui_lookup_widget() is
still present.
api export: Dynamic export of dialog functions
api export: Dynamic export of msgwin functions
api export: Dynamic export of encoding functions
api export: Dynamic export of keybindings functions
api export: Dynamic export of tagmanager functions

Do not add GEANY_EXPORT to foreign code, instead simply export everything as
in the old days.
api export: Dynamic export of search function
api export: Dynamic export of highlighting functions
api export: Dynamic export of filetypes functions
api export: Dynamic export of navqueue function
api export: Dynamic export of editor functions