Deduplicate code found in both egui_glium and egui_glow (#819)

* Move window building to egui-winit

* Move icon loading to egui-winit

* `use glow::HasContext;` -> `use glow::HasContext as _;`

* Move FileStorage into epi behind a feature flag

* De-duplicate screen_size_in_pixels and native_pixels_per_point

* Move creation of FileStorage to epi

* Handle epi app output (window size changes etc) in egui-winit

* Move app and memory persistence and autosave logic to egui-winit

* fix check.sh

* Make the epi backend opt-in for egui_glium and egui_glow

* Fix persistence

* Add integration name to epi::IntegrationInfo and the demo

* Clean up Cargo.toml files and fix making egui_glium optional

* fix typo

* Make egui_glium compile without the `epi` feature

Cargo.lock

@@ -837,6 +837,7 @@ name = "eframe"
 version = "0.14.0"
 dependencies = [
  "egui",
+ "egui-winit",
  "egui_glium",
  "egui_glow",
  "egui_web",
@@ -898,21 +899,17 @@ dependencies = [
 name = "egui_glium"
 version = "0.14.0"
 dependencies = [
- "directories-next",
  "egui",
  "egui-winit",
  "epi",
  "glium",
  "image",
- "ron",
- "serde",
 ]
 
 [[package]]
 name = "egui_glow"
 version = "0.14.0"
 dependencies = [
- "directories-next",
  "egui",
  "egui-winit",
  "epi",
@@ -920,8 +917,6 @@ dependencies = [
  "glutin",
  "image",
  "memoffset",
- "ron",
- "serde",
 ]
 
 [[package]]
@@ -1020,6 +1015,7 @@ dependencies = [
 name = "epi"
 version = "0.14.0"
 dependencies = [
+ "directories-next",
  "egui",
  "ron",
  "serde",