Add fixed_pos to Window (#16)

* Add fixed_pos to Window

This allows changing the position of a window by opening the window
with a different value.

* Add docstrings for `fixed_pos`

egui/src/containers/area.rs

@@ -90,6 +90,7 @@ impl Area {
         self
     }
 
+    /// Positions the window and prevents it from being moved
     pub fn fixed_pos(mut self, fixed_pos: impl Into<Pos2>) -> Self {
         let fixed_pos = fixed_pos.into();
         self.default_pos = Some(fixed_pos);

egui/src/containers/window.rs

@@ -91,6 +91,12 @@ impl<'open> Window<'open> {
         self.default_pos(rect.min).default_size(rect.size())
     }
 
+    /// Positions the window and prevents it from being moved
+    pub fn fixed_pos(mut self, pos: impl Into<Pos2>) -> Self {
+        self.area = self.area.fixed_pos(pos);
+        self
+    }
+
     pub fn fixed_size(mut self, size: impl Into<Vec2>) -> Self {
         self.resize = self.resize.fixed_size(size);
         self