This fixes a bunch of issues with dialogs on Windows.
- Removed lpstrFileTitle assignation, which overwrote the buffer
- Increased the memory size available for long file selections
- Removed seemingly unused `default_folder` in win_Args struct
- Properly handle the case where only one file is selected in multiselect mode
- Properly handle the initial folder, which would fail in specific conditions
The details for the last entry are explained in a comment in the code.
- Add a globally-accessible function to handle the parsing of filter extensions
- Remove the ability of putting the wildcard ('*') among other patterns; it's either a list of patterns or a single '*' now
- Add a hint to select between portals and Zenity on Unix
