Nimview

- Added "callFrontendJs" function to back-end to easilly call a javascript function (using Websockets in debug mode)
- Changed svelte examples to speed-up build

- applied fixes so that the demo can run on Ubuntu 21.04:
- fixed webview.nim to define window.external.invoke on gtk
- added webview locally as subtree, so I the PR doesn't need to be merged to the original webview library
- updated webview to the latest C99 webview of go wails
- changed the way data urls are generated, in case that "inlined.html" is used as url, so that gtk can handle it
- updated js so that data urls can be detected on gtk (you need to update nimview when used in npm packages.json)
- added some additional experimental compile time flag "-d:webview" so that webview2 can be used (can compile, has probably a lot of issues)

Added:
- tests,
- bugfixes,
- delayed start of webview / server as additional parameter

- Fixed nimble file
- Fixed android example
- Misc fixes

Breaking changes:
- removed old backend-helper.js and re-added a new npm based helper nimview.js: run "npm install nimview" and use "import nimview" in js files, see examples
- changed signature of js calls: window.backend.<function>(<params>...).then(<action>)

Features:
- added additional signatures to allow functions with multiple parameter for "addRequest"
- added single-executable binaries that contain the ui, if a file dist/inlined.html is available
- changed "examples" to be project templates, available by, for example: "npx degit marcomq/nimview/examples/svelte myProject"

updated functions for python and added (weak) csrf check