fix: Fix ui.Entry.SetReadOnly() to take bool instead of c_int

[jthat]

When porting the control gallery example (#14) I ran into the following build error:

% zig build
install
└─ install control-gallery
   └─ zig build-exe control-gallery Debug native 1 errors
examples/control-gallery.zig:237:33: error: expected type 'c_int', found 'bool'
    open_file_entry.SetReadOnly(true);
                                ^~~~
src/ui.zig:597:45: note: parameter type declared here
    pub fn SetReadOnly(e: *Entry, readonly: c_int) void {
                                            ^~~~~

This change fixes ui.Entry.SetReadOnly() to take a bool, not a c_int.