Gradient tool improvements (#927)

* Reuse existing gradient

* Double click to insert gradient stop

* Add history states to the gradient tool

* Do trig in viewport space so it is actually perpendicular

* Sync tool options with active gradient

* Deleting points with delete key

* More tolerance on inserting points

document-legacy/src/document.rs

@@ -32,6 +32,12 @@ pub struct Document {
 	pub state_identifier: DefaultHasher,
 }
 
+impl PartialEq for Document {
+	fn eq(&self, other: &Self) -> bool {
+		self.state_identifier.finish() == other.state_identifier.finish()
+	}
+}
+
 impl Default for Document {
 	fn default() -> Self {
 		Self {