v1.0.0-alpha4

It's getting more stable!

• component: fix issue with props not being validated as expected
• component: many small concurrency fixes
• observer: notify subscribers immediately instead of waiting for next tick
• qweb fix: allow mixing body and expressions variables
• component: display better error message if render is empty
• component: fix various issues with mounting/unmounting components
• context: notify changes even if owner is not mounted
• qweb: add support for typeof operator
• component: propagate errors to caller
• components/qweb: fix scoping issue with t-model and t-foreach
• component: various fixes with destroying components
• component: throw error if t-component is not used on a node t.